Nutrient sensing in pigs: A review

You would think that voluntary feed intake in pigs is based on nutritional needs. But there is much more going on. Nutrient sensing and the cross-talk between tongue, gut and brain play significant roles in how much the animal eats.

A high feed intake in young pigs is important to obtain maximum growth, hence leading to a better return on investment for the farmer. Foremost, the diet should contain all the nutrients the animal needs to grow and stay healthy. Young piglets often get a smaller pellet / mashed feed, full with highly digestible ingredients. However, over the last decades many studies have been carried out into how to stimulate feed intake by looking closer into the sensory profiles, the pig’s taste and the animals’ preferences. A chronicle review on this topic is presented here.

The understanding of the pig’s taste and feed intake mechanisms experienced a significant boost with the start of the genomic era. Photo: Shutterstock

From wild animals to livestock

If we look in nature, wild animals have a great sense of what to eat and what not to. Some foods want to be eaten because they contain energy and protein or have a pleasant odour or taste. Some plants have defence mechanisms to prevent the fruits, seeds and nuts (which are often part of the reproductive system) from being consumed and one of the preferred strategies across plant species has been related to bitter taste (not liked by animals). This so-called ‘chemosensory system’ in determining eating behaviour is widely accepted in wild animal species, but not implemented to a great extent in farm animals. In the case of pigs, they often are fed a single diet, with the assumption that animals eat whatever is in front of them as long as it is nutritionally balanced. But only the most limiting essential nutrients are balanced in a commercial pig diet. It is not well known how excess of nutrients may affect the pig’s appetite and dietary choice.

Large tongue and many taste buds

Already more than 100 years ago, researchers hypothesised that there is a relationship between taste and feed intake in pigs. The earliest traceable publications on pig taste relate to descriptive anatomy of the sensory system (1888 and 1922). Years later, in the early 90s, researchers discovered that pigs have a relatively large tongue with one of the highest number of taste buds (19,904) in mammalian species. Research in pig science really took off in the middle of the 20th century, when the American pig industry was starting to flourish. The first in-vivo pig studies took place on pig taste and feed intake in the 50s and 60s. This led to the concept of creep feeding in suckling piglets amongst others. Many studies involved sucrose and /or human high intensity sweeteners such as saccharin. This was also done in studies relating to human nutrition, as pigs were often used as a model. Some studies showed however that pigs show limited perception of human high intensity sweeteners. The next step in the academic world was to look at the relevance of gut sensing. Studies in the 70s showed that the upper gastrointestinal tract releases hormones that in turn determine short-term feed intake. More recently, it was also found that long term exposure to sucrose and maltodextrins reduces feed intake and increases the natural preferences for protein solutions. Research work from the 80s further delved into gut nutrient sensing and found that exogenously administered cholecystokinin (CCK) can have an influence on the feed intake in pigs. High concentrations of CCK are normally found in the brains of vertebrate species and it has been hypothesised that it plays a role in the control of appetite. This work was continued with more studies on the effect of CCK, fatty acid sensing and high intensity sweetener rebaudioside (non-caloric sugar replacer) on the release of gut hormones (incretins) and their effect on appetite.

Amino acids and overall flavour

Next to sugar or sugar replacers, also other ingredients and their effects on feed intake in pigs have been studied. Since the 90s, researchers found that supplementation of amino acids (both essential and non-essential) is important in relation to feed intake and preference. The essential amino acids for swine are lysine, methionine, threonine and tryptophan. In addition, preferences for potential umami taste by pigs have also received attention more recently. Umami, or savoury taste, is one of the five basic tastes (together with sweetness, sourness, bitterness, and saltiness). Overall, these works have illustrated the potential hedonic aspects of some non-essential amino acids (such as glutamine) together with the relevance of offering a balanced diet particularly regarding the most limiting amino acids. However, a clear relationship between amino acids and pig appetite has not been shown so far.

A more integrated approach on the impact of taste in the overall flavour of feeds was assessed in studies between 2009 and 2014, establishing pig preferences for a wide range of cereals, pulses and fats. One of the highlights was the preferences of highly digestible starch and simple carbohydrates (sugars) as well as for specific protein source.

More insights through genomics

The understanding of the pig’s taste and feed intake mechanisms experiences a significant boost with the start of the genomic era (knowledge of the structure, function, evolution, and mapping of DNA). The adoption of genomic tools has led to the discovery of the molecular mechanisms of taste and nutrient sensing and the detection of several receptor genes in several tissues of the digestive system. For example, the location of the calcium sensing receptor (CsSR) on the pig’s chromosome was discovered in the late 90s. Also nucleic acids encoding porcine taste receptors for porcine sweet and umami have been characterised. Umami and other amino acid receptors in the tongue have been identified, as well as fatty acid sensors. The latter is an important discovery to understand fat accreditation and growth in pigs. Receptors have also been identified outside of the mouth, such as in the small intestine and stomach. Nutrient sensing cells in the gastrointestinal tract have been involved in the secretion of gut hormones and other physiological functions, but existing data in pigs is scarce and how these mechanisms may affect feed intake remains largely unknown. In addition, the concept of olfactory systems in pigs is a relatively new and an uncovered field. Olfaction is a chemoreception that forms the sense of smell. It is known that pigs have a great ability to smell, but the effect on the brain and hence feed intake or feed preferences is not well understood yet.

More tools for researchers

From the many studies done over the years, it can be seen that early work primarily focused on the effects of sugars and artificial sweeteners. Research has evolved into emerging nutritional chemosensory science, including neuroscience. Overall, the onset of the genomic era brought researchers more tools to test the effect of certain feed ingredients at cellular level, which helps to better understand why certain compounds have an effect on feed intake and preferences of the animal. For example, now that we know that dietary protein levels affects the expression of the receptor pT1r1 in the post-weaning stage. This can help nutritionists to adapt the feed formulation or ingredient choice for certain age groups in growing pigs. More (genomic) research will follow to unlock the full potential of the science behind nutrient sensing. This will include more work on finding similarities between the taste receptor repertoire of humans and pigs, which will probably also increase the use of the pig as human model for nutritional studies.